This is a national stage of PCT application No. PCT/FI00/00191, filed on Mar. 10, 2000. Priority is claimed on that application, and on patent application No. 990558 filed in Finland on Mar. 12, 1999.
The present invention relates to a method and apparatus for manufacturing calendered and coated paper. The base paper is calendered using a multi-nip calender, for example, a super calender, an OptiLoad calender, or a Janus Concept calender.
The invention concerns the manufacture of high-quality printing papers, using on-line calendering and coating. In on-line solutions, the coating stations and calender are located on the same line at the end of the paper machine, the paper web manufactured being led directly from the paper machine to finishing, without intermediate winding. Calendered paper grades are manufactured using off-line equipment, so that two or three separate calenders, each with its own unwinder and reel-up, are used with a single paper machine. The speed of known multi-nip calenders has prevented their use as on-line calenders with high-speed paper machines. However, new multi-nip calenders and coating stations have been developed, which run at speeds that can be raised to match those of paper machines, allowing them to be connected directly to the same production line as a paper machine. All multi-nip calenders have several nips, which usually comprise hard and soft rolls. The surface of the soft rolls is made from paper or some other suitable fibrous material, or, to an increasing extent nowadays, from a polymer material developed for this purpose. The hard rolls are generally manufactured from cast iron and occasionally from steel, and can usually be heated using oil, steam, or in some other way, for instance, by using induction heating.
Calendering is intended to improve the gloss, smoothness, and other properties of the surface of the paper relating to the printability of the paper. These properties affect the final print quality.
Smoothness for the print impression is created by subjecting the paper fibres simultaneously to heat and high pressure, by heating hard rolls and pressing the rolls against against each other with great force, to create a high delay pressure in the nip between them. Due to these forces, the paper fibres reach their glass transition temperature, so that the deformations due to the nip load become permanent. The slipping of the paper against the surfaces of the rolls may also increase the deformations in the paper and the smoothing effect.
When multi-nip calendering is used, the paper is usually manufactured in a paper machine and, if necessary, coated. In both cases, the uncoated or coated paper is usually wound onto a batch roll and calendered in a separate calender. The paper is dried to become extremely dry, with a moisture content of about 1-3% of the total weight of the paper, the paper then being re-wet before calendering to a relatively high moisture content of about 6-10%. The purpose of the drying to a low moisture content and re-wetting is to even the cross-direction (CD) moisture profile. This method is used especially in the manufacture of super-calendered, i.e. SC paper. The short period of storage on the batch roll evens variations in moisture, in the same way as re-wetting. In present-day on-line calendering, the paper is dried to a very low moisture content and is re-wet to a suitable moisture content for calendering, immediately before calendering. The method is thus nearly identical to that used in off-line calendering, except that the storage that evens the moisture content is not used.
Re-wetting can be carried out using, for example, the waterjet units disclosed in U.S. Pat. No. 5,286,348, which create a good moisture profile in the cross direction of the web.
Problems arising from drying and re-wetting include the time required for the moisture content to even out and the increased energy required to evaporate the water needed for wetting. The greater drying requirement increases the length of the machine and the space needed for it, compared to equipment in which wetting is not required. An uneven moisture content will lead to variations in smoothness and in the thickness profile, because the moisture content has a great effect on the deformation of the fibres. If the thickness profile is uneven, winding becomes difficult and transverse buckling may even occur in customer reels. This buckling reduces the paper""s runnability in the preparing and printing machines and thus reduces the quality of the end product in the eyes of the customer.
Nowadays, the moisture profile of paper during manufacture is controlled in several ways, especially at the start of the formation of the web. In present processes, the control of the moisture profile is intended to ensure good runnability of the machine and the product being manufactured. This is understandable, because the tension profile, which greatly affects runnability, depends greatly on the moisture profile. In off-line calendering, the aim is to keep the moisture profile as even as possible, in those parts of the process in which it has the greatest effect on runnability.
A reason for coating paper is to improve its printability. Coating is used to influence the whiteness of the paper, the evenness of its surface, and the gloss of the print surface. Thus, coating has partly the same objectives as calendering. How thick a coating layer is used, what coating mixes are suitable, and how many times the paper is coated all depend on the use of the paper. The coating mix can be spread onto the surface of the paper in many different ways, each one of which creates a different final result, and surface on the end product. Usually, thicker coating and more coating layers will lead to a better print surface, so that, in art-paper grades, there may be several coating layers and a total coating amount of several tens of grams per square metre on each side of the paper. In lightweight coated printing (LWC) papers, the amount of coating on one side of the paper is significantly smaller, usually about 5-15 g/m2. Smaller amounts of coating do not necessarily cover the entire surface of the paper, instead the coating remains in the valleys formed in the surface roughness of the paper by the action of the doctor blade or other evening device. Thus, the coating thickness varies according to the roughness profile of the paper, so that the properties of the print surface are not absolutely even. However, the printing properties of LWC paper are better than those of corresponding uncoated paper, so that its cheaper price makes it quite suitable for uses requiring a reasonably good print surface at a low price. Naturally, the properties of LWC paper depend decisively on the type and quality of the base paper, and on the amount and type of coating used. Because a small amount of coating will not greatly increase whiteness, the base paper itself must be sufficiently white. Thus, LWC base paper usually contains chemical pulp fibres, which are intended not only to increase strength, but also to improve whiteness. Due to its characteristics, LWC paper is not usually calendered, as calendering cannot substantially influence the properties of this paper grade.
Paper based on mechanical pulp containing a great deal of filler can only be coated, if the filler content is sufficiently low. Base paper manufactured from mechanical pulp is weaker than paper made from chemical wood-free pulp and the addition of fillers further reduces its strength. For example, even under favourable conditions, the maximum filler content that can be used in blade coating is 15%. Thus, paper at least partly based on mechanical pulp and containing a large amount of filler has not been manufactured.
Another problem relating to coated papers, and particularly those containing filler, is recyclability. If the paper is recycled for fibre, the filler and coating must be removed, which demands a great deal of energy and especially strong chemicals. If this type of paper is finally used as a fuel, the coating and fillers remain as unburned ash, which hinders combustion and which is difficult to exploit economically. The amount of ash is also affected by the quality of fibre in the base paper. Though pulp will burn almost completely, groundwood fibres contain incombustible substances, which increase the amount of ash.
The invention is intended to create an entirely new type of method, by means of which it is possible to manufacture a new type of paper, and achieve, at lower cost, at least the same quality of print surface as in present paper grades intended for corresponding uses, especially LWC paper grades.
The invention is based on the calendering the manufactured paper web in a multi-nip calender while wet and then coating the calendered paper with a very small amount of coating. The resulting paper preferably has a layer of coating on at least one side with a maximum grammage of 5 g/m2.
Considerable advantages are gained with the aid of the invention.
The invention can be used to produce paper of a quality corresponding to LWC paper, with lower production costs, by very lightly coating a base paper produced from groundwood fibre. The invention can be used to manufacture coated paper from a base paper of low basic strength, because calendering carried out according to the invention increases the strength of paper. By optimizing the process, it is even possible to achieve a higher quality. The method can be used to reduce the price of high-quality lightweight-coated printing paper to an entirely new level. Because the total amount of coat is reduced, the amount of substance to be separated in recycling is reduced, compared to coated paper made form a corresponding base paper. Because the amount of coating is small, the grammage of the base paper can be correspondingly greater, allowing the use of fibres providing a lower strength, or a large amount of filler, without decreasing the paper""s strength or runnability. If it is possible to use groundwood fibre and a large amount of filler instead of pulp fibre, the price of the base paper will remain low, while, when using thin coating layers, the price of the coating will also be low. Because only a small amount of coating is used, and its dry substance content can be high, the dryer output required is significantly less than in the manufacture of LWC, for example. Thanks to the reduction in dryer output, the machine can be shortened and the energy consumption per tonne of paper manufactured reduced.
If multi-nip calendering takes place before coating, an extremely smooth paper surface is obtained before coating and the same smoothness is transferred to the properties of the coated paper. The calendered surface absorbs less water and the processed fibres expand less. Thus, thanks to precalendering, there is no substantial fibre roughening during coating, which would increase the roughness of the surface after coating. This property is extremely important when using a thin coating, because a thin coating layer will not cover possible roughening as well. In the same way, the small amount of water contained in a very thin coating layer will reduce the absorption of water by the fibres, accelerating drying and shortening the absorption time of the water.
While the invention is preferably applied in on-line processes, making the entire manufacturing line compact, it is also possible to apply the invention to solutions, in which the manufacturing of the base paper, calendering, and coating take place in different stages. However, in such cases, it may be difficult to control aspects such as the moisture content of the paper going to calendering, without additional wetting, which will increase costs. In other ways too, off-line manufacturing procedures are usually more expensive than on-line methods.
Due to the small amount of coating, little wetting of the base paper is needed, so that runnability remains good, as the wetting does not decrease the strength of the base paper. Also the paper fibres only expand slightly, partly due to the effect of calendering and partly due to the small amount of water absorbed.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are intended solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims.